1. Field of the Invention
This invention relates to the processing of data acquired by a measurement while drilling (MWD) tool during the drilling of a wellbore. More particularly, the invention relates to methods and devices for transmitting to the surface in real-time an image of the formation penetrated by the borehole as the borehole is being drilled using MWD telemetry.
2. State of the Art
Modern well drilling techniques, particularly those concerned with the drilling of oil and gas wells, involve the use of several different measurement and telemetry systems to provide petrophysical data and data regarding drilling mechanics during the drilling process. Data is acquired by sensors located in the drill string near the bit and either stored in downhole memory or transmitted to the surface using MWD telemetry devices. The state of the art in data acquisition and telemetry is well exemplified by the "Integrated Drilling Evaluation and Logging System" (IDEAL, a registered trademark of Schlumberger) developed by Schlumberger/Anadrill, Sugar Land, Texas. The IDEAL system comprises several components which provide useful information regarding downhole conditions at or near the bit while drilling. These components include a GEOSTEERING component which is disclosed in U.S. patent application Ser. Nos. 08/150,932 and 08/150,941, a RAB (resistivity-at-the-bit) component which is disclosed in U.S. patent application Ser. No. 07/955,101 and U.S. Pat. No. 5,235,285, a RWOB (receiver, weight on bit and torque) component which is disclosed in U.S. patent application Ser. No. 08/150,944, a POWERPULSE (MWD) telemetry component which is disclosed in U.S. Pat. Nos. 5,237,540 and 5,249,161 and in U.S. patent application Ser. No. 07/934,137, a CDR (compensated dual resistivity) component which is disclosed in U.S. Pat. No. 4,899,122, a CDN (compensated density neutron) component which is disclosed in U.S. Pat. No. 4,879,463, and an IDEAL Wellsite Information System. In addition, novel procedures for determining the instantaneous depth ("microdepth") and the rate of penetration of the drill bit are disclosed in U.S. Pat. No. 4,843,875.
At the outset, it should be noted that not all of the data acquired downhole is transmitted to the surface using MWD telemetry. MWD telemetry involves the mechanical pulsing of mud surrounding the drill string in the wellbore. Typically, downhole data is transmitted to the surface through mud pulse telemetry at a rate of approximately 1 bit per second. With the POWERPULSE MWD telemetry component, however, a continuous mud wave or "siren-type" telemetry method is used together with design features and software which enable a transmission rate of 6 or more bits per second. The POWERPULSE MWD component is also equipped with sensors which measure magnetic and gravitational fields in three dimensions. The magnetic and gravitational information can then be used to calculate inclination, azimuth, and toolface (instantaneous angular position) of the MWD component and thus the drill string. Since most of the downhole sensing components are capable of acquiring data much faster than can be transmitted by MWD telemetry, the data is typically stored in downhole memory for later downloading when the tool is tripped out of the borehole. Only selected data is transmitted to the surface in real-time for analysis while drilling.
The POWERPULSE MWD component is typically located in the drill string near the bit with the other mentioned components located either above or below it, depending on the particular component. Data acquired by the other components may be transmitted to the POWERPULSE MWD component by means of the RWOB component by wireless telemetry such as electromagnetic telemetry or ultrasonic telemetry. Those skilled in the art will appreciate that electromagnetic telemetry max provide a data transmission rate as high as a few hundred bits per second. Therefore, while efficiency of communication between the various downhole sensors and the MWD component is not as much an issue as the efficiency of communication between the MWD component and the surface, it is nevertheless a limiting factor in gathering real-time data.
Presently, the various components mentioned above acquire data regarding bit displacement including weight, torque, speed, inclination and azimuth, formation resistivities, gamma ray, density, and neutron porosity measurements. Of particular interest to the present invention are the data acquired by the RAB component which includes formation resistivity data. Presently, these data are used to evaluate the formation and for detecting fractures or thin or permeable beds. Azimuthal button electrodes on the RAB component acquire high resolution resistivity data. The resistivity measurements are indicative of the type of formation (mineral and porosity) present around the wellbore, e.g., sand, clay, lignite, montmorillonite, water, bound water, gas, and oil, each of which have a different resistivity. The resistivity data can be converted into image data based on the known resistive properties of formation components to create a color image where each resistivity is assigned a different color. Such an image will visually show the different layers of the formation and thereby aid in directing the drilling towards a hydrocarbon reservoir or in the production of hydrocarbons after the drilling has been completed. It is known to create such images with downloaded data after the tool is tripped out of the borehole. It is also known to create such images in real-time with a "wireline" imaging tool which transmits to the surface by cable. The known real-time imaging is therefore not performed while drilling, but is performed with a special imaging tool after drilling is completed.
Resistivity measurements for imaging are typically made in the range of 0.2-2,000 ohm-meters, i.e. a relative range of 10.sup.4. It would therefore take at least 13 bits to transmit a single resistivity measurement having a sensitivity of 0.2 ohm-meters. Consequently, it is impractical to transmit more than a few resistivity measurements to the surface using the MWD component. Instead, they are stored in downhole memory for later downloading when the tool is tripped out of the borehole. Some resistivity data can be transmitted to the surface by the MWD component and at the surface to the IDEAL Wellsite Information System which combines the resistivity data with surface data to provide a real-time numeric or chart display (a log) of the formation. While the log of the formation is extremely useful, the resolution of the real-time log is severely limited by the transmission rate of the MWD component.
As mentioned above, the most significant limitation of all real-time measurements in the wellbore is the data transmission rate of the mud pulse telemetry systems. Even with the state of the art 6 or more bits per second transmission rate, however, transmission of real-time formation image data is unthinkable. In order to create such an image, one must associate the toolface (instantaneous angular position of the tool) with the formation resistivity measurements from the RAB component and with the tool depth (actually the translational distance of the tool in the wellbore since the wellbore is not always vertical) at virtually exactly the same time (within a few milliseconds). Although the translational distance of the tool is available at the surface and the toolface and the formation measurements are ultimately available at the surface, they are not available within milliseconds of each other. Therefore, they must be associated downhole before transmission by the MWD component. However, as noted above, communication between the MWD component and the RAB component is at best a few hundred bits per second, not fast enough to associate data from both components within a few milliseconds. Even if the toolface data could be timely associated with the formation measurements and the translational distance data within a few milliseconds, the amount of data needed to define each distance-toolface-resistivity coordinate would be enormous in comparison to the slow transmission rate imposed by the MWD component.